In the distant past, drilling fluid containment systems for well drilling operations were typically accomplished by means of mud pits dug in the ground in the immediate vicinity of the drilling rig. Drilling fluid, typically referred to as drilling mud, emerging from the well being drilled would typically be directed into the mud pits. Larger solid particulate, such as drill cuttings, would settle immediately and the remaining drilling fluid, with much of its required solid particulate constituents, would be directed toward the suction of the mud pumps. The mud pumps simply would draw drilling fluid from the mud pits and pump it back into the circulating system of the drilling rig. A significant amount of required constituents of the drilling fluid was typically lost along with the drill cuttings, sand and silt since the drilling mud was allowed to settle and not typically agitated during settling. It was necessary, therefore, to continuously replace a substantial quantity of lost solid particulate such as barite immediately prior to recirculation of the drilling fluid into the well.
Since the solid constituents of drilling fluid represent a significant expense, it became desirable to ensure that as much as the solid constituents as possible were retained for reuse. It was also desirable to accomplish well drilling operations with as little quantity of drilling fluid as was necessary, thereby further controlling the expense of drilling operations.
In the more recent past, therefore, the drilling industry has developed the use of portable mud containment and circulating systems incorporating a plurality of mud tanks which are interconnected by means of piping. These mud tanks are typically of rectangular configuration and are formed simply by welding metal plates in assembly. In order to develop turbulence in the tanks, impellers are typically provided in the central portion of the tank to generate turbulant flow radiating toward the bottom portion of the side walls of the tanks. In order to minimize the quantity of drilling fluid required for drilling operations, the drilling fluid emerging from the well is typically subjected to preliminary treatment for separation of larger solids before the drilling fluid is deposited in the first one of the mud tanks. For example, shale shakers are employed to separate drill cuttings by means of vibratory screening process. After removal of the drill cuttings, smaller particulate such as sand and silt are also be removed. The drilling fluid entering the first one of the mud tanks typically incorporates the usual drilling fluid constituents such as barite and other requisite components of the drilling mud along with a certain amount of sand and silt that is not removed by way of the preliminary screening treatment. The drilling fluid in the first one of the mud tanks is, therefore, considered contaminated and is unsuitable for use in further drilling operations without subsequent treatment. Subsequent treatment is typically accomplished by means of desander and desilter separators of centrifugal nature.
The mud tanks typically employed during drilling operations are required to contain sufficient drilling fluid for continuous circulation for normal drilling with a small amount of drilling fluid being continuously added to accommodate losses into the formation being drilled and to also accommodate the increasing dimension of the hole being drilled. It is extremely important to maintain sufficient quantity of drilling fluid to kill the well in the event the conditions of possible blowout are encountered. If a gas pocket is encountered and gas begins to enter the well bore, the hydrostatic head of the column of drilling fluid in the well bore will typically be reduced since the specific gravity of the heavy liquid drilling fluid is reduced by the presence of gas. To prevent a well blowout, it is typically necessary to pump drilling fluid rapidly into the bottom portion of the well bore to develop a hydrostatic head in the well bore which is in excess of the pressure of production fluid at the bottom of the well bore, thus preventing gas and other production fluid from entering the well bore. When rectangular drilling fluid containment systems are employed, sediment begins to develop at the corner portions of the tank where the side walls and bottom wall intersect because of the lack of fluid turbulence in these areas. This sediment buildup can severely reduce the volume of drilling fluid such that insufficient volume drilling fluid remains for adequate well control to prevent well blowout. To compensate for the buildup of sediment, conventional mud tanks generally have much more volumetric capacity than is actually required for drilling and for maintenance of a safe drilling fluid reserve. This excessive fluid volume adds cost and is, of course, detrimental to the commercial aspects of the drilling operation. Further, after drilling operations are terminated, the bottom and side wall portions of the drilling mud tanks sufficiently contain sufficient deposit of sediment that a significant amount of labor is required to remove it and thus render the mud tanks portable for transportation to other drilling sites. The volume of sediment in the bottom portion of drilling fluid tanks is typically of sufficient concern that the tanks must be checked quite often. Actual physical measurements must be taken to ensure that the remaining volumetric capacity of the drilling mud tanks is sufficient to contain the necessary volume of fluid for purposes of well safety. It is, of course, desirable to provide a drilling fluid containment and circulating system incorporating drilling fluid tanks which have the capability to remain free of any sediment, thereby ensuring that the tanks always contain sufficient volume of drilling fluid for purposes of well safety. It is also desirable that drilling fluid tanks be provided which do not require manual labor for sediment cleaning after well drilling operations have been terminated.
When impellers are utilized to agitate the drilling fluid in conventional rectangular mud tanks, a vortex typically develops which has the detrimental effect of severely restricting the volume of fluid circulation to the immediate central portion of the mud tank. In this case, only the center portion of the drilling fluid is agitated or circulated and dead spots develop in the peripheral portion of the tank, allowing settling. It is desirable to ensure turbulence through the drilling fluid volume in all areas of the mud tank to thus prevent or retard settling.